The present invention relates to a method and apparatus for the control of an ignition system spark timing for a spark ignition internal combustion engine, and more particularly to a method and apparatus for the control of an ignition system spark timing where not only fuel consumption and power output are required, but also purification of exhaust gases is required.
Conventionally, the spark timing control where the exhaust gas recirculation (hereinafter abbreviated as EGR) takes place, employs a two-way or three-way ON-OFF valve, often called as "TV valve," which in response to the engine coolant temperature effects on-off control of atmosphere passages leading to an EGR control unit and a spark timing control unit, respectively. (For example, if the atmosphere passages are open, EGR is suspended and the spark advance is also suspended.) FIG. 1 shows the open and close states of the TV valve, wherein the axis of abscissa designates the coolant temperature of the engine, the wave-form line (1) represents the open and close states of the atmosphere passage leading to the EGR control unit, and the wave-form line (2) represents the open and close states of the atmosphere passage to the spark timing control unit. From this it will be understood that when the coolant temperature is higher than 95.degree. C., the atmosphere passage to the EGR control unit is open to suspend EGR for the purpose of engine protection. When the coolant temperature is lower than 60.degree., the atmosphere passage to the EGR control unit is open to suspend EGR for the purpose of preventing deterioration in driveability. As will be understood from the wave-form line (2), when the coolant temperature is from 15.degree. C. to 60.degree. C. representing warm-up period of the engine, the atmosphere passage leading to the spark ignition control unit is open, thereby to suspend the spark advance control, suppressing the spark advance to a small value. This causes rapid warm-up and facilitates warm-up of the exhaust gas purifier. Within a range when the coolant temperature is lower than 15.degree. C., an increase in spark advance is allowed to provide a sufficient increase in spark advance for the purpose of preventing deterioration in driveability. If EGR is effected during a temperature range from 15.degree. C. to 60.degree. C. when spark advance control is suspended, the driveability drops excessively because driveability which has been worsened during this temperature range owing to the suspension of spark advance control is further worsened by the addition of EGR, and besides no appreciable effect in reducing NOx is seen because during this range the amount of NOx has already been suppressed as a result of suspension of spark advance control. This explains the reasons why, as shown by wave-form lines (1) and (2), it has been desired to use the same temperature for the beginning temperature of the EGR and for the resuming temperature of the spark advance control, wherein the EGR begins above 60.degree. C. and the spark advance control resumes above 60.degree. C.
For the reason set forth above, it has been the conventional practice to combine spark timing control with EGR control to effect a control in response to the engine coolant temperature.
However, in this conventional combination control, disabling and enabling of EGR and spark advance control are effected in response to coolant temperature and the correction of spark advance value is not effected in response to a rise or drop in the exhaust gas recirculation rate, so that even if EGR is suspended, the value of spark advance remains unchanged such as at a coolant temperature of 95.degree. C. shown in FIG. 1, thus causing knocking. The same phenomenon takes place too within a low temperature range. Besides, since the control is characterized by so called "two-values control" wherein there occurs a disabled-state or an abled-state, the value of spark advance can not correspond to the change even if minute EGR control is effected.